In order to get you CAC working you need to install - opensc - which you can get through the Synaptic Package Manager - if you do not have Synaptic installed open the terminal
Code:
sudo apt install synaptic
after installation then do a search for - opensc - it will find it you may have to scroll down a bit but it is there, it will also install opensc-pkcs11 - simply right click it and select "Mark for installation" then click the apply button it will then download it and install it
Once you get that installed I have made a guide on my website - it is at the bottom of the page that will take you through the rest process it is a PDF file - I had a CAC for several years as a contractor and retired USAF
Expirion Linux It's a great operating system with the XFCE Desktop
expirionlinux.wixsite.com
Yes the guide says for Expirion but it is based on Ubuntu so it should be the same
Hope this helps